Portable, handheld microanalytical systems, which have been termed “chemical laboratories on a chip,” are being developed to enable the rapid and sensitive detection of particular chemicals, including pollutants, high explosives, and chemical and biological warfare agents. Microfabricated analytical systems have the advantages of reduced power consumption and the ability to batch fabricate and assemble complete microanalytical systems on an integrated chip. In particular, increasing integration can reduce the lower limit of detectability by reducing dead volume and allowing for extremely short, heated transfer lengths between system components.
Current gas-phase microanalytical systems typically comprise a gas chromatography column to separate the chemical species, or analyte, in a gas mixture, and a detector to detect the separated species. Such microanalytical systems can also include a chemical preconcentrator. The chemical preconcentrator serves the important function of selectively collecting and concentrating the analyte(s) of interest out of a large gas sample volume on a sorptive material at the inlet of the microanalytical system. In particular, selective analyte preconcentration is an essential step for early-warning, trace chemical detection in real-world, high-consequence environments where a high background of potentially interfering compounds exists. The chemical preconcentrator can deliver an extremely sharp analyte plug to the downstream gas chromatograph by taking advantage of the rapid, efficient heating of the sorbed analyte with a low-heat capacity, low-loss microheater. The very narrow temporal plug improves baseline separations, and therefore the signal-to-noise ratio and detectability of the particular chemical species of interest. Further, with a rapid enough release, there is a greatly reduced need for mechanical means of sample introduction, such as valving. See R. P. Manginell et al., “Recent Advancements in the Gas-Phase MicroChemLab,” Proc. of SPIE 5591, 44 (2004).
Previous microfabricated chemical preconcentrators have used a heated planar membrane suspended from a substrate as the microheater, wherein the sorptive material is disposed as a layer on a surface of the membrane to sorb the analytes from a gas stream. See U.S. Pat. No. 6,171,378 to Manginell et al., which is incorporated herein by reference. The high thermal efficiency, extremely low heat capacity, and low flow impedance of the planar preconcentrator enables very rapid thermal desorption of the chemical analyte with very low power consumption. In particular, the desorption rate is rapid enough to eliminate the need for a separate mechanical column injection loop. However, analyte uptake on the sorptive layer is low, due to sorptive materials limitations and the low collection area of the sorptive layer of the planar preconcentrator. In particular, the total collection capacity is inadequate for application to volatile compounds and materials, due largely to the low capacity of the planar adsorbent zone.
A non-planar chemical preconcentrator has also been developed that uses a high-surface area, low mass, three-dimensional, flow-through support structure that can be coated or packed with a sorptive material. See U.S. patent application Ser. No. 10/696,649 to Manginell et al., which is incorporated herein by reference. Two basic styles have been implemented. One employs flow perpendicular to the substrate surface, through a cylindrical adsorbent coating structure fabricated in the bulk of a silicon chip. The other allows the sample to flow parallel to the substrate surface through parallel, fin-like adsorbent coating structures. The high-surface area of the sorption support structure allows improved analyte collection and concentration, especially important for trace chemical detection. In particular, the measured collection capacity for semivolatile compounds on sol-gel adsorbents is increased by an order of magnitude, compared to the planar preconcentrator. Lithographically defined gas flow constrictions within the support structure improve mass transfer of analyte into and out of the adsorbent, so that while the transient electrothermal response of the non-planar preconcentrator can be two orders of magnitude longer than the planar preconcentrator, due to the added support mass, desorbed analyte peak widths are only a factor of two wider. However, while very successful, the low tortuosity of the flow through the support structure of the prior non-planar preconcentrator is not optimal for certain analytes. This low tortuosity can be alleviated by using a packed bed, which has a high capacity and nearly 100% capture efficiency for analyte collection. However, the packed bed has a very high flow impedance, requires relatively high power consumption, and has limited thermal transfer to the bed.
The non-planar chemical preconcentrator of the present invention comprises a sorptive support structure having a tortuous flow path. The tortuous path increases contact of the analyte with the sorptive material by reducing the boundary layer width, as compared with the prior non-planar preconcentrator support structure. It also provides more opportunities for desorption and readsorption of volatile species. Under certain flow conditions, vortices of flow can be created, further enhancing analyte collection and desorption. Therefore, the tortuous path preconcentrator provides a compromise between a planar preconcentrator and a packed bed preconcentrator. Further, the thermal efficiency of the tortuous path chemical preconcentrator is comparable or superior to the prior non-planar chemical preconcentrator. Finally, the tortuosity can be varied in different directions to optimize flow rates during the adsorption and desorption phases of operation of the preconcentrator.